Precession photographs of crystalline Nuclease-T' are being taken at many levels of reciprocal lattice planes and the reflections thus recorded are being processed using a computer in order to obtain a difference electron density map between nuclease A and Nuclease-T. The extra amino acid sequence contained in nuclease B, a possible precursor of nuclease A, has been established to be: Ser-Glu-Thr-Asp-Asx-Gly-Val-Asx-Arg-Ser-Gly-Ser-Glu-Asp-Pro-Thr-Val-Tyr-Ser where the two residues indicated by Asx are Asp in nuclease B1, and Asn in nuclease B3 and the first and the second Asx from the NH2-terminus are Asp and Asn, respectively in nuclease B2. This extra sequence attached to the NH2-terminus of nuclease A. Saturation of Nuclease-(1-126) with ligands (p dt p and Ca ions) decreases its intrinsic viscosity from 6.59 to 5.23 cubic cm. The latter value for the intrinsic viscosity is equivalent to that of liganded nuclease B and clearly greater than that of liganded nuclease A. These results are compatible with the idea that liganded Nuclease-(1-126) may possess a conformation composed of ordered as well as flexible segments due to lack of cooperativity.